As part of mining/oil & gas exploration activities, as well as extracting rock samples for construction/civil engineering, there is a need to obtain underground ‘core’ samples for analysis by geologists.
Core orientation is the process of obtaining and marking the orientation of a core sample from a drilling operation. The orientation of the sample is determined with regard to its original position in a body of material, such as rock or ore deposits underground.
Such core samples are obtained by drilling into an underground medium, such as sedimentary rock, and extracting a solid cylindrical core which reveals, amongst other things, the type of rock, rock strata, presence or absence of minerals or other deposits, and any veins of useful deposits. Core samples can be correlated against each other to reveal trends in rock strata and deposits, which help predict whether mining is worthwhile, and if so, where, in what direction and how deep below the surface.
In order to obtain required information from the extracted core samples, a core orientation device is attached between a greaser unit and an inner core tube holding the core sample. The purpose of the core orientation device is to measure and log the orientation of the core with respect to the ‘down-side’ of the underground location from which it has been extracted. This is an important process as these core samples are used to build a three dimensional profile of existing subsurface resource deposits, such as iron ore or diamonds. If a valuable ore seam is found, it is vital that the core is orientated properly so that a true picture of the ore body can be developed underground.
Whilst depth and azimuth are used as important indicators of core position, they are generally inadequate on their own to determine the original position and attitude of subsurface geological features. Core orientation enables such details to be determined.
Orientation of the core sample needs to be obtained from a drilling operation. The orientation of the sample is determined with regard to its original position in a body of material, such as rock or ore deposits underground. Core orientation i.e. which side of the core was facing the bottom (or top) of a borehole and rotational orientation compared to surrounding material, enables such details to be determined.
Core samples are cylindrical in shape, typically around 3 meters long, and are obtained by drilling with an annular hollow core drill into subsurface material, such as sediment and rock, and recovering the core sample. A diamond tipped drill bit is used at the end of the hollow drill string. As the drill progresses deeper, more sections of hollow steel drill tube are added to extend the drill string. An inner tube assembly captures the core sample. This inner tube assembly remains stationary while the outer tubes rotate with the drill bit. Thus, the core sample is pushed into the inner tube.
Once retrieved to the surface, the core end is subsequently marked to indicate orientation of the core sample.
Current practice involves the core orientation being recorded during drilling, and analysis is undertaken during core logging. The core logging process requires the use of systems to measure the angles of the geological features, such as an integrated core logging system.
Through core orientation, it is possible to understand the geology of a subsurface region and from that make strategic decisions on future mining or drilling operations, such as economic feasibility, predicted ore body volume, and layout planning. In the construction industry, core orientation can reveal geological features that may affect siting or structural foundations for buildings.
Typical systems and methodologies presently used periodically record orientation of the core between commencement and end of drilling. Vibration from drilling causes many recorded orientation results to be inaccurrate or not needed because orientation before end of drilling is not required or used. This needless recordal of data wastes the limited power of the onboard battery powering the orientation sensors, and thereby limits the amount of time an orientation unit can remain downhole before needing a recharge or battery replacement.
Apart from analyzing this content of the core sample, it is also necessary to determine the ‘orientation’ of the core(s) with respect to the drilling angle and depth from the earth's surface and the direction of rotation of the core, at the source of extraction. These measurements are used as an aid in determining the consistency and direction of deposits, such as ore content, and for producing a 3D ‘picture’ of underground mineralization.
After retrieving the core sample to the surface, the core orientation device will then be used to electronically or mechanically determine the core's orientation before being drilled out. The operator would have to rotate the whole inner tube so as to position the core tube such that the core is set in an up/down position in the core tube. This gives a correct reference for the original orientation of the material in the core when it was attached to the ground material prior to extraction. The core sample end is then visually marked to show the correct up/down orientation for later analysis.
It has been realised that the methodology of obtaining the desired orientation of the core representative of the point at which the core was ‘broken’ away from the body from which it is drilled could be improved.
To this end, it has been found desirable of the present invention to provide a method and system of obtaining an indication of core orientation that reduces power demand on the orientation unit and avoids the need to record orientation data that is not needed. This aims to simplify and speed up the core orientation data gathering process.
Core orientation is recorded during drilling, and analysis is undertaken during core logging. The core logging process requires the use of systems to measure the angles of the geological features, such as an integrated core logging system.
Through core orientation, it is possible to understand the geology of a subsurface region and from that make strategic decisions on future mining or drilling operations, such as economic feasibility, predicted ore body volume, and layout planning. In the construction industry, core orientation can reveal geological features that may affect siting or structural foundations for buildings
In a drill string, a ‘back end’ assembly connects to a greaser. This greaser lubricates the back end assembly which rotates with the outer casing while the greaser remains stationary with the inner tubing.
Once a core sample is cut, the inner tube assembly is recovered by winching to the surface. After removal of the back end assembly from the inner tube assembly, the core sample is recovered and catalogued for analysis.
Various core orientation systems have previously been used or proposed. Traditional systems use a spear and clay impression arrangement where a spear is thrown down the drill string and makes an impression in clay material at an upper end of the core sample. This impression can be used to vindicate the orientation of the core at the time and position the spear impacted the clay.
A more recent system of determining core orientation is proposed in Australian patent number 2006100113 (also as U.S. Pat. No. 7,584,055). This patent document describes a core orientation device for a core drill. The device provides signals associated with a physical orientation of a core orientation device for a particular moment in time. The device includes a memory for storing and providing the orientation data when required. The system described in AU 2006100113 provides a two unit replacement for the greaser described above. A first orientation system unit houses electronics and a battery used to record orientation data, and the second greaser unit is an extended greaser accommodating a physical screw on connector for the first unit as well as serving as the greaser. This combination forms part of the inner tube assembly with the core tube, orientation system ‘first’ unit and the connector/greaser ‘second’ unit. However, as a result of the now extended length of the combined orientation system and greaser units compared with a standard greaser only unit, the outer drill string casing now requires a matching extension piece to extend the outer casing an equal amount. The core orientation system has a display on one face which is used when setting up the unit prior to deployment, and to indicate core sample alignment when the core sample is recovered. At the surface before removing the core sample from the inner tube assembly, the operator views the display fitted on the system. The display indicates for the operator to rotate the unit and the sample within the tube until the whole core tube and sample is oriented with the lower section of the core sample at the lower end of the tube. The core sample is marked (usually by pencil) before being removed from the core for future analysis.
However, the device described in AU 2006100113 has been found to have certain limitations. The orientation unit is connected to the greaser by a screw thread and o-ring seal arrangement. In the harsh down hole environment within the drill string, it has been realised that the o-ring seals are not always effective and can let fluid into the space between the orientation unit and the greaser. The display unit allows fluid into the electronics of the orientation, resulting in a risk of fault or failure of the device. Furthermore, the orientation unit must be disassembled from the greaser unit before the display and orientation unit can be viewed, rotated and the required core orientation displayed. Thus, the device of AU 2006100113 requires manual manipulation before any reading can be viewed on the display, if the display and the electronics have survived any ingress of fluid past the o-ring seal.
Furthermore, a problem has been identified in the known art. Battery powered downhole survey equipment, such as probes and core orientation units, are typically switched on at the surface and run almost continuously or operate on a frequent timer basis. For example, a known core orientation device the subject of Australian patent application AU 2010200162 takes measurements determined by a timer whether or not the values obtained are worthwhile or accurate. This leads a large amount of unusable data which is typically discarded and such continuous or too often recording of data unnecessarily rapidly reduces battery life of the downhole device. Such known arrangements may only last a few weeks or months before the downhole device needs recharging or replacing. Often spare equipment is held on hand just in case the batter fails. This leads to far too much equipment being needed, at an increased cost to the drilling operator. It would be beneficial to reduce reliance on holding spare equipment on hand.
In addition, it has been realised that, during the drilling process, if sections of fragmented earth are drilled into (resulting in fractured core samples) then the inner tube can rotate. Furthermore, vibrations caused by drilling have also been identified as a cause of inaccurate data.
Also, it has been realised that only a limited amount of downhole data is actually required in order to later determine correct orientation of a core sample at the surface. It has been realised that data recording on a continuous or frequent periodic basis whilst drilling is occurring is unnecessary. Only down orientation of the core sample needs to be known, and provided data relating to the down orientation can be identified and referenced to a particular known time, core orientation can be determined.
It has therefore been found desirable to provide improved downhole data recording through a system, device and method that alleviates one or more of the aforementioned problems whilst facilitating more reliable data recovery.
After retrieving the core sample to the surface, the core orientation device will then be used to electronically or mechanically determine the core's orientation before being drilled out. The operator would have to rotate the whole inner tube so as to position the core tube such that the core is set in an up/down position in the core tube. This gives a correct reference for the original orientation of the material in the core when it was attached to the ground material prior to extraction.
Personnel then physically ‘mark’ the lower end position of that core sample end face protruding from the core tube with a wax pencil or similar marker (usually a red wax pencil). In order to accurately mark the ‘lower end’ of the core face, a device is used to determine the position to mark the core. This is usually achieved with the aid of spirit-level v-block devices to determine the position to place the ‘lower-end’ mark on the core face.
This procedure, although straightforward, is often carried out incorrectly, leading to incorrect marking of the orientation of the core. This error is often due to insufficient training, lack of understanding due to language barriers, operator fatigue, ineffectually carrying out of the procedure or basic v-groove spirit level devices not being used correctly or their correct use not being easily understood.
Incorrect marking of the core orientation through human error leads to poor geophysical analysis and results. It has been found that geologists, on realising the marking error, have needed to search through core samples and determine the correct orientation. This loses many man hours of work in having to go back through the original core samples and identify the correct orientation, and until this is done, further development of the worksite cannot be accurately carried out. Mining may commence or continue in the wrong place and/or may miss the vein of resource.
With the aforementioned in mind, it is desirable of the present invention to provide improved means and way by which core sample orientation can be accurately marked.